When a patient is unable to breathe unaided, or requires assistance with breathing, the patient is usually connected to an artificial respiratory circuit including a ventilator programmed by a clinician to deliver an appropriate volume of air, or an air/oxygen mixture, to the patient. In such a respiratory circuit, it is desirable to prevent the patient from exhaling fully, and therefore the patient's lungs from deflating fully. This is because complete deflation, and subsequent reflation, of the patient's lungs requires a significant amount of the patient's energy.
Prevention of total exhalation is generally achieved by including a mechanism in the respiratory circuit which only allows exhaled breath above an appropriate exhalation pressure to escape the respiratory circuit through an exhalation port. Prevention of total exhalation in this way is known as applying “PEEP” to the respiratory circuit, where “PEEP” refers to Positive End Expiratory Pressure.
PEEP is currently applied to a respiratory circuit using either a so-called PEEP valve or an exhalation valve to control the passage of the exhaled breath through an exhalation port. A PEEP valve has a fixed and pre-determined release pressure for the exhalation port. An exhalation valve has a release pressure that is determined by the pressure of a gas within a control chamber of the exhalation valve. This gas within the control chamber is usually supplied by the ventilator at a pressure suitable to apply the desired positive end expiratory pressure to the respiratory circuit.
Conventionally, exhalation valves comprise a control chamber that is supplied by the ventilator, during use, with a gas under pressure, and a flexible membrane that defines a wall of the chamber and is disposed adjacent to a valve seat surrounding the exhalation port of the respiratory circuit. The flexible membrane is typically formed by a diaphragm or a balloon valve member.
In use, gas is supplied to the control chamber by the ventilator, and the supplied gas deforms the membrane elastically and outwardly from the chamber and into engagement with the valve seat, thereby sealing the exhalation port. The positive end expiratory pressure to be applied to the respiratory circuit is selected at the ventilator, and the ventilator supplies an appropriate pressure of gas to the control chamber to achieve that positive end expiratory pressure.
Different types of ventilator require there to be a different ratio between the pressure within the control chamber of the exhalation valve, and the airway pressure required to open the valve. This ratio is determined by the exhalation valve dimensions, including the diameter of the valve seat that surrounds the exhalation port, which means that different exhalation valves need to be used with different ventilators.
It is also necessary for the valve seat to be manufactured with a small tolerance in its diameter to achieve the desired accuracy in the pressure applied by the ventilator. However, the use of materials that are mouldable with small tolerances is expensive, and many such materials do not have the desired properties for other functions of the valve, eg low friction for use with a swivel connector.
The present invention overcomes or substantially mitigates some or all of the above mentioned and/or other disadvantages of the prior art.